Internally illuminated signs present an image to the viewer which generally is more easily observed at night than non-illuminated or externally illuminated signs. Internally illuminated signs are commonly used in outdoor advertising. As the primary function of traffic signs is to present important information to automobile drivers efficiently, internally illuminated signs are also useful in this application.
However, internally illuminated signs are not suitable for night use where electricity is not readily available (such as traffic signs for remote rural roads), or in any location where loss of power is possible (i.e., if the sign is connected to an interruptable circuit without a backup power source). Another problem is that the light sources (typically incandescent or fluorescent bulbs) require periodic replacement due to their limited life. Retroreflective materials have been used to augment internally illuminated signs so they will be visible when illuminated by a vehicle headlight. In this way, the sign is visible even if the power or light source is not operative.
One type of retroreflective material generally comprises microspheres with a specular reflecting layer added to reflect the incident light within a narrow angular range about the incident light direction. An embodiment of this approach embeds the microspheres in strands of a web. The application of this to an internally illuminated sign utilizes the open areas of the web for transmission of the internal illumination to the viewer of the sign, while the retroreflective microspheres allow the sign to be viewed at night under vehicle headlights.
Another type of retroreflective material comprises cube corner elements. As taught by U.S. Pat. No. 4,588,258 (Hoopman), such sheeting may be made to transmit internal illumination by separating cube corner elements, producing flat interstitial areas which transmit light from inside the sign to the outside.